Function decide 

pub fn decide(
    frac: f64,
    current: f64,
    high: f64,
    low: f64,
    floor: f64,
    ceiling: f64,
    last_bad: Option<f64>,
) -> Decision

Pure governor step. frac is the windowed attempt-failure fraction, current the committed offer, last_bad the offer at which congestion was last observed (None = unexplored — slow start).